Tutorial – Converting GRAMS to MOLES to ATOMS or
MOLECULES

Using the grid & bridge concepts(Std
3inCh 8)

bridge 1bridge 2bridge 3

milligrams<--->grams<--->moles<--->atoms or molecules

Bridges are fractions that equal 1.They allow
us to change one unit of measure to another, like crossing a bridge over a
river from one side to the other.Notice that each bridge can go in two directions, just like a real
bridge over a river.For a bridge
to work, the top and bottom of the fraction must equal each other .That is why the value of all bridges = 1.

The
following equivalent equations will be made into bridges (fractions that = 1) to
do go back and forth from one unit of measure to another:

Your job is to create fractions
that will take you the way you need to go.The example will use both the bridge concept and conversion grids.Here’s the example:

Example 1

You are given 850 mg ofCa and asked to
find out how many atoms that represents.Each bridge is made as follows:

Solution – Ex 1

bridge 11000 mg = 1 g

bridge 240.1 g Ca=1
mole Ca

bridge 31 mole=6.02
x 1023 atoms

We make each
fraction from the bridge that takes us from one unit of measure to
another.The side of the fraction that
goes on top, is the one with unit of measure we are
changing to.Here is how it
works:

starting
fraction:850 mg/1 (the number one)ending fraction:atoms/1

bridge 1bridge 2bridge 3

850 mg Ca

1 g

1 mol Ca

6.02 x 1023atoms

1

1000 mg

40.1 g Ca

1 mol Ca

Now let’s see
what the grid looks like when we cancel units:

850 mgCa

1 g

1 mol Ca

6.02 x 1023Ca atoms

1

1000 mg

40.1 gCa

1 mol Ca

The final
fraction looks like this:

5,117 x 1023Ca atoms

40,100

Now all we have
to do is divide the two numbers at the end:

0.128 x 1023 Ca atoms

For correct scientific notation we must move the
decimal one place to the right.When we
do that we must adjust the exponent of 10 to be one number less, or we change
the value of the number.

FINAL
ANSWER =1.28 x 1022Ca atoms

Example 2

You are given4.35 x 1019 atoms of
Mg.You are supposed to find how many
grams that represents.

Solution

First let’s see
what the grid looks like:

bridge
1bridge 2bridge 3

4.35 x 1019 atoms Mg

1 mole

24.3 g Mg

1

6.20 x 1023 atoms

1 mole Mg

Now let’s cancel
units and see what is left:

4.35 x 1019atoms Mg

1 mole

24.3 g Mg

1

6.02 x 1023atoms

1 mole Mg

This is equal
to:

105.7 x 1019 g Mg

6.02 x 1023

As in Example 1, we now
divide the numbers.This time though we
do the “normal” numbers separately from the 10’s with the exponents:

105.7 / 6.02 =17.61019/1023=10-4

Now we
“reassemble” the numbers to read

17.6 x 10-4

This time to change to scientific notation we must
move the decimal to the left one place.In order to keep the same number we must adjust the exponent of 10 to be
3.

FINAL
ANSWER =1.76 x 10-3

If this didn’t make sense to you, go back over
it slowly a few times and let it “sink in”.